Do anticancer agents reach the tumor target in the human brain?

Re-Irradiation by Stereotactic Radiotherapy of Brain Metastases in the Case of Local Recurrence

PURPOSE

To evaluate the efficacy and safety of a second course of stereotactic radiotherapy (SRT2) treatment for a local recurrence of brain metastases previously treated with SRT (SRT1), using the Hypofractionated Treatment Effects in the Clinic (HyTEC) reporting standards and the European Society for Radiotherapy and Oncology guidelines.

METHODS

From December 2014 to May 2021, 32 patients with 34 brain metastases received salvage SRT2 after failed SRT1. A total dose of 21 to 27 Gy in 3 fractions or 30 Gy in 5 fractions was prescribed to the periphery of the PTV (99% of the prescribed dose covering 99% of the PTV). After SRT2, multiparametric MRI, sometimes combined with 18F-DOPA PET-CT, was performed every 3 months to determine local control (LC) and radionecrosis (RN).

RESULTS

After a median follow-up of 12 months (range: 1-37 months), the crude LC and RN rates were 68% and 12%, respectively, and the median overall survival was 25 months. In a multivariate analysis, the performance of surgery was predictive of a significantly better LC (p = 0.002) and survival benefit (p = 0.04). The volume of a normal brain receiving 5 Gy during SRT2 (p = 0.04), a dose delivered to the PTV in SRT1 (p = 0.003), and concomitant systemic therapy (p = 0.04) were associated with an increased risk of RN.

CONCLUSION

SRT2 is an effective approach for the local recurrence of BM after initial SRT treatment and is a potential salvage therapy option for well-selected people with a good performance status. Surgery was associated with a higher LC.

Intracranial germ cell tumors in Adolescents and Young Adults: European and North American consensus review, current management and future development

The incidence of intracranial germ cell tumors (iGCT) is much lower in European and North American (E&NA) than in Asian population. However, E&NA cooperative groups have simultaneously developed with success treatment strategies with specific attention paid to long-term sequelae. Neurological sequelae may be reduced by establishing a diagnosis with an endoscopic biopsy and/or cerebrospinal fluid (CSF) and/or serum analysis, deferring the need to perform a radical surgery. Depending on markers and/or histological characteristics, patients are treated as either germinoma or non-germinomatous germ cell tumors (NGGCT). Metastatic disease is defined by a positive CSF cytology and/or distant drops in craniospinal MRI. The combination of surgery and/or chemotherapy and radiation therapy is tailored according to grouping and staging. With more than 90% 5-year event-free survival (EFS), localized germinomas can be managed without aggressive surgery, and benefit from chemotherapy followed by whole ventricular irradiation with local boost. Bifocal germinomas are treated as non-metastatic entities. Metastatic germinomas may be cured with craniospinal irradiation. With a 5-year EFS over 70%, NGGCT benefit from chemotherapy followed by delayed surgery in case of residual disease, and some form of radiotherapy. Future strategies will aim at decreasing long-term side effects while preserving high cure rates.

The structure of blood-tumor barrier and distribution of chemotherapeutic drugs in non-small cell lung cancer brain metastases

Background

Brain metastasis is an important cause of increased mortality in patients with non-small cell lung cancer (NSCLC). In brain metastasis, the blood–brain barrier (BBB) is frequently impaired, forming blood–tumor barrier (BTB). The efficacy of chemotherapy is usually very poor. However, the characteristics of BTB and the impacts of BTB on chemotherapeutic drug delivery remain unclear. The present study investigated the structure of BTB, as well as the distribution of routine clinical chemotherapeutic drugs in both brain and peripheral tumors.

Methods

Bioluminescent image was used to monitor the tumor load after intracranial injection of lung cancer Lewis cells in mice. The permeability of BBB and BTB was measured by fluorescent tracers of evans blue and fluorescein sodium. Transmission electron microscopy (TEM), immunohistochemistry and immunofluorescence were performed to analyze structural differences between BBB and BTB. The concentrations of chemotherapeutic drugs (gemcitabine, paclitaxel and pemetrexed) in tissues were assayed by liquid chromatography with tandem mass spectrometry (LC-MS/MS).

Results

Brain metastases exhibited increased BTB permeability compared with normal BBB detected by fluorescence tracers. TEM showed abnormal blood vessels, damaged endothelial cells, thick basement membranes, impaired intercellular endothelial tight junctions, as well as increased fenestrae and pinocytotic vesicles in metastatic lesions. Immunohistochemistry and immunofluorescence revealed that astrocytes were distributed surrounded the blood vessels both in normal brain and the tumor border, but no astrocytes were found in the inner metastatic lesions. By LC-MS/MS analysis, gemcitabine showed higher permeability in brain metastases.

Conclusions

Brain metastases of lung cancer disrupted the structure of BBB, and this disruption was heterogeneous. Chemotherapeutic drugs can cross the BTB of brain metastases of lung cancer but have difficulty crossing the normal BBB. Among the three commonly used chemotherapy drugs, gemcitabine has the highest distribution in brain metastases. The permeability of chemotherapeutic agents is related to their molecular weight and liposolubility.

Pharmacokinetic Principles and Their Application to Central Nervous System Tumors

Despite increasing knowledge of the biologic drivers of central nervous system tumors, most targeted agents trialed to date have not shown activity against these tumors in clinical trials. To effectively treat central nervous system tumors, an active drug must achieve and maintain an effective exposure at the tumor site for a long enough period of time to exert its intended effect. However, this is difficult to assess and achieve due to the constraints of drug delivery to the central nervous system. To address this complex problem, an understanding of pharmacokinetic principles is necessary. Pharmacokinetics is classically described as the quantitative study of drug absorption, distribution, metabolism, and elimination. The innate chemical properties of a drug, its administration (dose, route and schedule), and host factors all influence these four key pharmacokinetic phases. The central nervous system adds a level of complexity to standard plasma pharmacokinetics as it is a coupled drug compartment. This review will discuss special considerations of pharmacokinetics in the context of therapeutic development for central nervous system tumors.

Systemic therapy for brain metastases

Metastases from cells outside of the central nervous system are the most common cancer found in the brain and are commonly associated with poor prognosis. Although cancer treatment is improving overall, central nervous system metastases are becoming more prevalent and require finesse to properly treat. Physicians must consider the biology of the primary tumor and the complex neurological environment that the metastasis resides in. This can be further complicated by the fact that the practice of cancer management is constantly evolving and therapy that works outside of the blood-brain barrier may not be effective inside of it. Therefore, this review seeks to update the reader on recent advancements made on the three most common sources of brain metastases: lung cancer, breast cancer, and melanoma. Each of these malignancies has been the subject of intriguing and novel avenues of therapy which are reviewed here.

The effect of locally delivered cisplatin is dependent on an intact immune function in an experimental glioma model

Several chemotherapeutic drugs are now considered to exert anti-tumour effects, by inducing an immune-promoting inflammatory response. Cisplatin is a potent chemotherapeutic agent used in standard medulloblastoma but not glioblastoma protocols. There is no clear explanation for the differences in clinical efficacy of cisplatin between medulloblastomas and glioblastomas, despite the fact that cisplatin is effective in vitro against the latter. Systemic toxicity is often dose limiting but could tentatively be reduced by intratumoral administration. We found that intratumoral cisplatin can cure GL261 glioma-bearing C57BL/6 mice and this effect was abolished in GL261-bearing NOD-scid IL2rγnull (NSG) mice. Contrary to previous results with intratumoral temozolomide cisplatin had no additive or synergistic effect with whole cell either GL261 wild-type or GM-CSF-transfected GL261 cells whole cell vaccine-based immunotherapy. While whole tumour cell immunizations increased CD8+ T-cells and decreased F4/80+ macrophages intratumorally, cisplatin had no effect on these cell populations. Taken together, our results demonstrate that intratumoral cisplatin treatment was effective with a narrow therapeutic window and may be an efficient approach for glioma or other brain tumour treatment.

Pre-irradiation intensive induction and marrow-ablative consolidation chemotherapy in young children with newly diagnosed high-grade brainstem gliomas: report of the "head-start" I and II clinical trials

BACKGROUND

The dismal outcome in children with high-grade brainstem gliomas (BSG) accentuates the need for effective therapeutic strategies. We investigated the role of intensive, including marrow-ablative, chemotherapy regimens in the treatment of young children with newly-diagnosed high-grade BSG.

METHODS

Between 1991-and-2002, 15 eligible children less than 10 years of age with a diagnosis of high-grade BSG were treated on "Head-Start" I and II protocols (HSI and HSII). Treatment included Induction with 4-5 cycles of one of three intensive chemotherapy regimens followed by Consolidation with one cycle of marrow-ablative chemotherapy (thiotepa, carboplatin and etoposide) with autologous hematopoietic cell rescue (AHCR). Irradiation was required for children over 6 years of age or for those with residual tumor at the end of Consolidation.

RESULTS

We had two long-term survivors who were found retrospectively to harbor low-grade glial tumors and thus were not included in the survival analysis. Of the remaining 13 patients, the 1-year event-free (EFS) and overall (OS) survival for these children were 31% (95% CI 9-55%) and 38% (95% CI 14-63%), respectively. Median EFS and OS were 6.6 (95% CI 2.7, 12.7) and 8.7 months (95% CI 6.9, 20.9), respectively. Eight patients developed progressive disease during study treatment (seven during Induction and one at the end of Consolidation). Ten children received focal irradiation, five for residual tumor (three following Induction and two following Consolidation) and five due to disease progression.

CONCLUSIONS

Children with high-grade BSG did not benefit from this intensive chemotherapy strategy administered prior to irradiation.

Immune Checkpoint Inhibitors for the Treatment of Central Nervous System (CNS) Metastatic Disease

While the CNS has long been viewed as an immune-privileged environment, a paradigm shift in neuro-immunology has elevated the role of systemic immunotherapy for the treatment of metastatic disease. Increasing knowledge regarding the presence of a CNS lymphatic system and the physical and biochemical alteration of the blood brain barrier (BBB) by the tumor microenvironment suggests immune cell trafficking in and out of the CNS is possible. Emerging clinical data suggest immune checkpoint inhibitors (ICIs) can stimulate T cells peripherally to in turn have anti-tumor effects in the CNS. For example, anti-programmed cell death-1 (PD-1) monotherapy with pembrolizumab has shown intracranial response rates of 20-30% in patients with melanoma or non-small cell lung cancer (NSCLC) brain metastases. The combination of nivolumab and ipilimumab [anti-PD-1 and anti-cytotoxic T-lymphocyte-associated protein 4 (CTLA-4)] showed an intracranial response rate of 55% in patients with melanoma brain metastases. More data are needed to confirm these response rates and to determine mechanisms of efficacy and resistance. While local therapies such as stereotactic radiosurgery (SRS), whole-brain radiation therapy (WBRT), and surgery remain current mainstays, ICIS offer potential decreased neurotoxicity. This review summarizes the biological rationale for systemic immunotherapy to treat CNS metastatic disease, existing clinical data on ICIs in this setting and ongoing clinical trials exploring areas of unmet need.

Incomplete copolymer degradation of in situ chemotherapy

In situ carmustine wafers containing 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) are commonly used for the treatment of recurrent glioblastoma to overcome the brain-blood barrier. In theory, this chemotherapy diffuses into the adjacent parenchyma and the excipient degrades in maximum 8 weeks but no clinical data confirms this evolution, because patients are rarely operated again. A 75-year-old patient was operated twice for recurrent glioblastoma, and a carmustine wafer was implanted during the second surgery. Eleven months later, a third surgery was performed, revealing unexpected incomplete degradation of the wafer. 1H-Nuclear Magnetic Resonance was performed to compare this wafer to pure BCNU and to an unused copolymer wafer. In the used wafer, peaks corresponding to hydrophobic units of the excipient were no longer noticeable, whereas peaks of the hydrophilic units and traces of BCNU were still present. These surprising results could be related to the formation of a hydrophobic membrane around the wafer, thus interfering with the expected diffusion and degradation processes. The clinical benefit of carmustine wafers in addition to the standard radio-chemotherapy remains limited, and in vivo behavior of this treatment is not completely elucidated yet. We found that the wafer may remain after several months. Alternative strategies to deal with the blood-brain barrier, such as drug-loaded liposomes or ultrasound-opening, must be explored to offer larger drug diffusion or allow repetitive delivery.

Expected Paradigm Shift in Brain Metastases Therapy-Immune Checkpoint Inhibitors

Brain metastasis (BM) is one of the dreadful complications of malignancies. The prognosis after BM is extremely poor and life expectancy is meager. Currently, our treatment modalities are limited to radiotherapy and surgical resection, which also has poor outcomes and leads to various neurological deficits and affects the quality of life of patients. New treatment modality, i.e., immune checkpoint inhibitors, has brought revolution in management of melanoma, renal cancer, and non-small cell lung cancer (NSCLC). Immune checkpoint inhibitors basically enhance the immune response of the body to fight against cancers. Immune response in the brain is highly regulated; therefore, it is challenging to use immune-modulator drugs in BM. The microenvironment of BM is rich in cytotoxic T lymphocytes and which is the target of immune checkpoint inhibitors. Few studies have shown some hope regarding use of immune checkpoint inhibitors in management of BM. It works through inhibiting immune check point gates, i.e., CTLA-4 (cytotoxic T-lymphocyte-associated protein) and PD-1/PD-L1 (programmed cell death protein-1/program death ligand-1). This article explains the basic mechanism of immune check point inhibitors, rationale behind their usage in BM, and some of the clinical studies which have shown the efficacy of immune check point inhibitors in BM.

Targeted concurrent and sequential delivery of chemotherapeutic and antiangiogenic agents to the brain by using drug-loaded nanofibrous membranes

Glioblastoma is the most frequent and devastating primary brain tumor. Surgery followed by radiotherapy with concomitant and adjuvant chemotherapy is the standard of care for patients with glioblastoma. Chemotherapy is ineffective, because of the low therapeutic levels of pharmaceuticals in tumor tissues and the well-known tumor-cell resistance to chemotherapy. Therefore, we developed bilayered poly(d,l)-lactide-co-glycolide nanofibrous membranes that enabled the sequential and sustained release of chemotherapeutic and antiangiogenic agents by employing an electrospinning technique. The release characteristics of embedded drugs were determined by employing an in vitro elution technique and high-performance liquid chromatography. The experimental results showed that the fabricated nanofibers showed a sequential drug-eluting behavior, with the release of high drug levels of chemotherapeutic carmustine, irinotecan, and cisplatin from day 3, followed by the release of high concentrations of the antiangiogenic combretastatin from day 21. Biodegradable multidrug-eluting nanofibrous membranes were then dispersed into the cerebral cavity of rats by craniectomy, and the in vivo release characteristics of the pharmaceuticals from the membranes were investigated. The results suggested that the nanofibrous membranes released high concentrations of pharmaceuticals for more than 8 weeks in the cerebral parenchyma of rats. The result of histological analysis demonstrated developmental atrophy of brains with no inflammation. Biodegradable nanofibrous membranes can be manufactured for long-term sequential transport of different chemotherapeutic and anti-angiogenic agents in the brain, which can potentially improve the treatment of glioblastoma multiforme and prevent toxic effects due to systemic administration.

Developmental therapeutics for patients with breast cancer and central nervous system metastasis: current landscape and future perspectives

Breast cancer is the second-leading cause of metastatic disease in the central nervous system (CNS). Recent advances in the biological understanding of breast cancer have facilitated an unprecedented increase of survival in a subset of patients presenting with metastatic breast cancer. Patients with HER2 positive (HER2+) or triple negative breast cancer are at highest risk of developing CNS metastasis, and typically experience a poor prognosis despite treatment with local and systemic therapies. Among the obstacles ahead in the realm of developmental therapeutics for breast cancer CNS metastasis is the improvement of our knowledge on its biological nuances and on the interaction of the blood–brain barrier with new compounds. This article reviews recent discoveries related to the underlying biology of breast cancer brain metastases, clinical progress to date and suggests rational approaches for investigational therapies.

Triolein Emulsion Infusion Into the Carotid Artery Increases Brain Permeability to Anticancer Agents

BACKGROUND

Triolein emulsion infusion into the carotid artery has been reported to induce temporary and reversible opening of the blood-brain barrier by increasing vascular permeability.

OBJECTIVE

To evaluate the effect of triolein emulsion infusion on brain permeance by anticancer agents.

METHODS

In the doxorubicin study. 2.4 mg/kg doxorubicin was injected immediately after triolein emulsion (1%, 1.5%, and 2%) infusion into rabbit carotid arteries. Two hours later, bilateral hemispheres and eyeballs were harvested, and doxorubicin concentrations were measured fluorometrically. Doxorubicin ratios of ipsilateral/contralateral hemispheres were compared with those of doxorubicin controls by use of the Kruskal-Wallis test followed by the Dunn test. In the cisplatin study, 10 mg/kg cisplatin was injected immediately after 2% triolein emulsion infusion into rat carotid arteries. Ipsilateral hemispheres were harvested 2, 6, 12, 24, and 36 hours after treatment. Time-dependent cisplatin concentrations were determined by liquid chromatography/electrospray ionization-tandem mass spectrometry/mass spectrometry.

RESULTS

Doxorubicin concentrations were significantly higher in ipsilateral hemispheres and eyeballs in all 3 triolein treatment groups than in doxorubicin controls. In the cisplatin study, cisplatin concentrations in the ipsilateral hemispheres peaked at 6 hours after infusion of cisplatin.

CONCLUSION

Brain permeance to anticancer agents was increased by triolein emulsion infusion, which suggests that triolein infusion might be a useful adjuvant treatment for brain tumors.

Safe long-term repeated disruption of the blood-brain barrier using an implantable ultrasound device: a multiparametric study in a primate model

OBJECTIVE The main limitation to the efficacy of chemotherapy for brain tumors is the restricted access to the brain because of the limited permeability of the blood-brain barrier (BBB). Previous animal studies have shown that the application of pulsed ultrasound (US), in combination with the intravenous injection of microbubbles, can temporarily disrupt the BBB to deliver drugs that normally cannot reach brain tissue. Although many previous studies have been performed with external focused US transducers, the device described in the current work emits US energy using an unfocused transducer implanted in the skull thickness. This method avoids distortion of the US energy by the skull bone and allows for simple, repetitive, and broad disruption of the BBB without the need for MRI monitoring. The purpose of the present study was to determine if the BBB can be safely and repeatedly disrupted using such an implantable unfocused US device in a primate model. METHODS An 11.5-mm-diameter, 1-MHz, planar US device was implanted via a bur hole into the skull of 3 primates (2 Papio anubis [olive] baboons and 1 Macaca fascicularis [macaque]) for 4 months. Pulsed US sonications were applied together with the simultaneous intravenous injection of sulfur hexafluoride microbubbles (SonoVue) every 2 weeks to temporarily disrupt the BBB. In each primate, a total of 7 sonications were performed with a 23.2-msec burst length (25,000 cycles) and a 1-Hz pulse repetition frequency at acoustic pressure levels of 0.6-0.8 MPa. Potential toxicity induced by repeated BBB opening was analyzed using MRI, PET, electroencephalography (EEG), somatosensory evoked potential (SSEP) monitoring, behavioral scales, and histopathological analysis. RESULTS The T1-weighted contrast-enhanced MR images acquired after each sonication exhibited a zone of hypersignal underneath the transducer that persisted for more than 4 hours, indicating a broad region of BBB opening in the acoustic field of the implant. Positron emission tomography images with fluorine-18-labeled fluorodeoxyglucose (FDG) did not indicate any changes in the cerebral metabolism of glucose. Neither epileptic signs nor pathological central nerve conduction was observed on EEG and SSEP recordings, respectively. Behavior in all animals remained normal. Histological analysis showed no hemorrhagic processes, no petechia, and extravasation of only a few erythrocytes. CONCLUSIONS The studies performed confirm that an implantable, 1-MHz US device can be used to repeatedly open the BBB broadly in a large-animal model without inducing any acute, subacute, or chronic lesions.

Disease characteristics and survival outcomes of extragonadal primary germ cell tumour in two Canadian tertiary cancer centres

INTRODUCTION

Extragonadal germ cell tumours (EGCTs) are a heterogeneous group with distinct natural history and responses to treatment modalities. We sought to evaluate characteristics and survival outcomes in men with EGCTs.

METHODS

We performed a retrospective analysis on a consecutive list of men diagnosed with EGCT in two Albertan cancer centres between 1990 and 2013. Demographic characteristics and outcomes, stratified by primary site, were evaluated.

RESULTS

Sixty-nine cases were identified. The median age was 29 (range 15-76) and 48 cases (70%) were non-seminomatous. Twenty-four (35%) belonged to International Germ Cell Cancer Collaborative Group (IGCCCG) favourable risk group, 14 (20%) to intermediate, and 31 (45%) to poor. Thirty (43%) had mediastinal primary (MPs); 29 were treated with first-line bleomycin, etopo-side, and cisplatin (BEP). Seventeen (57%) relapses occurred, of which three patients achieved long-term survival. Seventeen (25%) had a central nervous system (CNS) primary, with eight (47%) classic germinoma. Seven (41%) received primary chemotherapy alone; 5 (29%) received primary radiotherapy alone, and 5 (29%) received both. Nineteen (28%) had a retroperitoneal primary (RPs) and received first-line chemotherapy; all but two received BEP and eight (42%) had surgical resection. Three (5%) had other or unknown primary. Five-year overall survival (OS) and disease-free survival for all patients were 56% and 44%, respectively; for MPs, 44% and 34%; for CNS primary, 76% and 53%; for RPs, 58% and 53%. Factors that correlated with decreased OS were elevated alpha fetoprotein (AFP) (p<0.001) or human chorionic gonadotropin (HCG) (p=0.001), lactate dehydrogenase (LDH) levels (p=0.028), bone metastasis (p<0.001), lung metastasis (p<0.001), and IGCCCG poor risk (p=0.001).

CONCLUSIONS

EGCT is a rare, but important subset of GCT. Patients with EGCTs, despite aggressive treatments, still have poorer outcomes than gonadal primary.

Phase II Trial of Temsirolimus for Relapsed/Refractory Primary CNS Lymphoma

PURPOSE

In this phase II study (NCT00942747), temsirolimus was tested in patients with relapsed or refractory primary CNS lymphoma (PCNSL).

PATIENTS AND METHODS

Immunocompetent adults with histologically confirmed PCNSL after experiencing high-dose methotrexate-based chemotherapy failure who were not eligible for or had experienced high-dose chemotherapy with autologous stem-cell transplant failure were included. The first cohort (n = 6) received 25 mg temsirolimus intravenously once per week. All consecutive patients received 75 mg intravenously once per week.

RESULTS

Thirty-seven eligible patients (median age, 70 years) were included whose median time since their last treatment was 3.9 months (range, 0.1 to 14.6 months). Complete response was seen in five patients (13.5%), complete response unconfirmed in three (8%), and partial response in 12 (32.4%) for an overall response rate of 54%. Median progression-free survival was 2.1 months (95% CI, 1.1 to 3.0 months). The most frequent Common Toxicity Criteria ≥ 3° adverse event was hyperglycemia in 11 (29.7%) patients, thrombocytopenia in eight (21.6%), infection in seven (19%), anemia in four (10.8%), and rash in three (8.1%). Fourteen blood/CSF pairs were collected in nine patients (10 pairs in five patients in the 25-mg cohort and four pairs in four patients in the 75-mg cohort). The mean maximum blood concentration was 292 ng/mL for temsirolimus and 37.2 ng/mL for its metabolite sirolimus in the 25-mg cohort and 484 ng/mL and 91.1 ng/mL, respectively, in the 75-mg cohort. Temsirolimus CSF concentration was 2 ng/mL in one patient in the 75-mg cohort; in all others, no drug was found in their CSF.

CONCLUSION

Single-agent temsirolimus at a weekly dose of 75 mg was found to be active in relapsed/refractory patients with PCNSL; however, responses were usually short lived.

Salinomycin encapsulated nanoparticles as a targeting vehicle for glioblastoma cells

Salinomycin has been introduced as a novel alternative to traditional anti-cancer drugs. The aim of this study was to test a strategy designed to deliver salinomycin to glioblastoma cells in vitro. Salinomycin-encapsulated polysorbate 80-coated poly(lactic-co-glycolic acid) nanoparticles (P80-SAL-PLGA) were prepared and characterized with respect to particle size, morphology, thermal properties, drug encapsulation efficiency and controlled salinomycin-release behaviour. The in vitro cellular uptake of P80-SAL-PLGA (5 and 10 µM) or uncoated nanoparticles was assessed in T98G human glioblastoma cells, and the cell viability was investigated with respect to anti-growth activities. SAL, which was successfully transported to T98G glioblastoma cells via P80 coated nanoparticles (∼14% within 60 min), greatly decreased (p < 0.01) the cellular viability of T98G cells. Substantial morphological changes were observed in the T98G cells with damaged actin cytoskeleton. Thus, P80-SAL-PLGA nanoparticles induced cell death, suggesting a potential therapeutic role for this salinomycin delivery system in the treatment of human glioblastoma.

Concurrent delivery of carmustine, irinotecan, and cisplatin to the cerebral cavity using biodegradable nanofibers: In vitro and in vivo studies

Glioblastoma multiforme (GBM) is the most common primary malignant brain tumor, and the prognosis of patients afflicted with GBM has been dismal, exhibiting progressive neurologic impairment and imminent death. Even with the most active regimens currently available, chemotherapy achieves only modest improvement in the overall survival. New chemotherapeutic agents and novel approaches to therapy are required for improving clinical outcomes. In this study, we used an electrospinning technique and developed biodegradable poly[(d,l)-lactide-co-glycolide] nanofibrous membranes that facilitated a sustained release of carmustine (or bis-chloroethylnitrosourea, BCNU), irinotecan, and cisplatin. An elution method and a high-performance liquid chromatography assay were employed to characterize the in vitro and in vivo release behaviors of pharmaceuticals from the nanofibrous membranes. The experimental results showed that the biodegradable, nanofibrous membranes released high concentrations of BCNU, irinotecan, and cisplatin for more than 8 weeks in the cerebral cavity of rats. A histological examination revealed progressive atrophy of the brain tissues without inflammatory reactions. Biodegradable drug-eluting nanofibrous membranes may facilitate sustained delivery of various and concurrent chemotherapeutic agents in the cerebral cavity, enhancing the therapeutic efficacy of GBM treatment and preventing toxic effects resulting from the systemic administration of chemotherapeutic agents.

Tumor bed radiosurgery following resection and prior stereotactic radiosurgery for locally persistent brain metastasis

PURPOSE

Despite advances in multimodality management of brain metastases, local progression following stereotactic radiosurgery (SRS) can occur. Often, surgical resection is favored, as it frequently provides immediate symptom relief as well as pathological characterization of any residual tumor. Should the pathological specimen contain viable tumor cells, further radiation therapy is an option to sterilize the tumor bed. We evaluated the use of repeat SRS (rSRS) in lieu of whole-brain radiation therapy (WBRT) as a means of improving local control (LC) while minimizing potential toxicity and dose to the normal brain.

MATERIALS/METHODS

A retrospective review was performed to identify patients with brain metastases who underwent SRS and then surgical resection for locally recurrent or persistent disease. From 2004 to 2014, 13 consecutive patients or 15 lesions were treated with rSRS after resection, either post-operatively to the tumor bed (n = 10, 66.6%) or after a second local recurrence (n = 5, 33.3%). LC, distant brain failure (DBF), and radiation toxicity were determined using patient records, RECIST criteria v1.1, and CTCAE v4.03.

RESULTS

At a median follow-up interval of 9.0 months (range 1.8-54.9 months) from time of rSRS, five patients remain alive. Following rSRS, 13 of the 15 (86.6%) lesions were locally controlled with an estimated 100% LC at 6 months and 75% LC at 1 year. However, 11 of the 15 (73.3%) treated lesions developed DBF after rSRS with 3 of 13 patients proceeding to WBRT. Two of 15 (13.3%) resulted in either grade 2 radionecrosis with grade 3 seizures or grade 3 radionecrosis.

CONCLUSION

Repeat SRS represents a potential salvage therapy for patients with locally recurrent brain metastases, providing additional tumor control with acceptable toxicity, even in the setting of prior SRS and surgical resection. rSRS may be reasonable to use as an alternative to WBRT in this setting.

Current strategies for targeted delivery of bio-active drug molecules in the treatment of brain tumor

Brain tumor is one of the most challenging diseases to treat. The major obstacle in the specific drug delivery to brain is blood-brain barrier (BBB). Mostly available anti-cancer drugs are large hydrophobic molecules which have limited permeability via BBB. Therefore, it is clear that the protective barriers confining the passage of the foreign particles into the brain are the main impediment for the brain drug delivery. Hence, the major challenge in drug development and delivery for the neurological diseases is to design non-invasive nanocarrier systems that can assist controlled and targeted drug delivery to the specific regions of the brain. In this review article, our major focus to treat brain tumor by study numerous strategies includes intracerebral implants, BBB disruption, intraventricular infusion, convection-enhanced delivery, intra-arterial drug delivery, intrathecal drug delivery, injection, catheters, pumps, microdialysis, RNA interference, antisense therapy, gene therapy, monoclonal/cationic antibodies conjugate, endogenous transporters, lipophilic analogues, prodrugs, efflux transporters, direct conjugation of antitumor drugs, direct targeting of liposomes, nanoparticles, solid-lipid nanoparticles, polymeric micelles, dendrimers and albumin-based drug carriers.

Blood brain barrier: a challenge for effectual therapy of brain tumors

Brain tumors are one of the most formidable diseases of mankind. They have only a fair to poor prognosis and high relapse rate. One of the major causes of extreme difficulty in brain tumor treatment is the presence of blood brain barrier (BBB). BBB comprises different molecular components and transport systems, which in turn create efflux machinery or hindrance for the entry of several drugs in brain. Thus, along with the conventional techniques, successful modification of drug delivery and novel therapeutic strategies are needed to overcome this obstacle for treatment of brain tumors. In this review, we have elucidated some critical insights into the composition and function of BBB and along with it we have discussed the effective methods for delivery of drugs to the brain and therapeutic strategies overcoming the barrier.

Brain metastasis: new opportunities to tackle therapeutic resistance

Brain metastasis is a devastating complication of cancer with unmet therapeutic needs. The incidence of brain metastasis has been rising in cancer patients and its response to treatment is limited due to the singular characteristics of brain metastasis (i.e., blood-brain-barrier, immune system, stroma). Despite improvements in the treatment and control of extracranial disease, the outcomes of patients with brain metastasis remain dismal. The mechanisms that allow tumor cells to promulgate metastases to the brain remain poorly understood. Further work is required to identify the molecular alterations inherent to brain metastasis in order to identify novel therapeutic targets and explicate the mechanisms of resistance to systemic therapeutics. In this article, we review current knowledge of the unique characteristics of brain metastasis, implications in therapeutic resistance, and the possibility of developing biomarkers to rationally guide the use of targeted agents.

Drug delivery to the central nervous system by polymeric nanoparticles: what do we know?

Nanoparticles enable the delivery of a great variety of drugs including anticancer drugs, analgesics, anti-Alzheimer's drugs, cardiovascular drugs, protease inhibitors, and several macromolecules into the brain after intravenous injection of animals. The mechanism of the nanoparticle-mediated drug transport across the BBB appears to be receptor-mediated endocytosis followed by transcytosis into the brain or by drug release within the endothelial cells. Modification of the nanoparticle surface with covalently attached targeting ligands or by coating with certain surfactants that lead to the adsorption of specific plasma proteins after injection is necessary for this receptor-mediated uptake. A very critical and important requirement for nanoparticulate brain delivery is that the employed nanoparticles are biocompatible and, moreover, rapidly biodegradable, i.e. over a time frame of a few days. In addition to enabling drug delivery to the brain, nanoparticles, as with doxorubicin, may importantly reduce the drug's toxicity and adverse effects due to an alteration of the body distribution. Because of the possibility to treat severe CNS diseases such as brain tumours and to even transport proteins and other macromolecules across the blood-brain barrier, this technology holds great promise for a non-invasive therapy of these diseases.

Therapeutic nanomedicine for brain cancer

Malignant brain cancer treatment is limited by a number of barriers, including the blood-brain barrier, transport within the brain interstitium, difficulties in delivering therapeutics specifically to tumor cells, the highly invasive quality of gliomas and drug resistance. As a result, the prognosis for patients with high-grade gliomas is poor and has improved little in recent years. Nanomedicine approaches have been developed in the laboratory, with some technologies being translated to the clinic, in order to address these needs. This review discusses the obstacles to effective treatment that are currently faced in the field, as well as various nanomedicine techniques that have been used or are being explored to overcome them, with a focus on liposomal and polymeric nanoparticles.

Modeling mass transfer from carmustine-loaded polymeric implants for malignant gliomas

Significant advances in the encapsulation and release of drugs from degradable polymers have led to the Food and Drug Administration approval of Gliadel wafers for controlled local delivery of the chemotherapeutic drug carmustine to high-grade gliomas following surgical resection. Due to the localized nature of the delivery method, no pharmacokinetic measurements have been taken in humans. Rather, pharmacokinetic studies in animals and associated modeling have indicated the capability of carmustine to be delivered in high concentrations within millimeters from the implant site over approximately 5 days. Mathematical models have indicated that diffusion has a primary role in transport, which may be complemented by enhanced fluid convection from postsurgical edema in the initial 3 days following implantation. Carmustine's penetration distance is also presumably limited by its lipophilicity and permeability in the capillaries. This review discusses the mathematical models that have been used to predict the release and distribution of carmustine from a polymeric implant. These models provide a theoretical framework for greater understanding of systems for localized drug delivery while highlighting factors that should be considered in clinical applications. In effect, Gliadel wafers and similar drug delivery implants can be optimized with reduction in required time and resources with such a quantitative and integrative approach.

Intratumoral modeling of gefitinib pharmacokinetics and pharmacodynamics in an orthotopic mouse model of glioblastoma

Like many solid tumors, glioblastomas are characterized by intratumoral biologic heterogeneity that may contribute to a variable distribution of drugs and their associated pharmacodynamic responses, such that the standard pharmacokinetic approaches based on analysis of whole-tumor homogenates may be inaccurate. To address this aspect of tumor pharmacology, we analyzed intratumoral pharmacokinetic/pharmacodynamic characteristics of the EGFR inhibitor gefitinib in mice with intracerebral tumors and developed corresponding mathematical models. Following a single oral dose of gefitinib (50 or 150 mg/kg), tumors were processed at selected times according to a novel brain tumor sectioning protocol that generated serial samples to measure gefitinib concentrations, phosphorylated extracellular signal-regulated kinase (pERK), and immunohistochemistry in 4 different regions of tumors. Notably, we observed up to 3-fold variations in intratumoral concentrations of gefitinib, but only up to half this variability in pERK levels. As we observed a similar degree of variation in the immunohistochemical index termed the microvessel pericyte index (MPI), a measure of permeability in the blood-brain barrier, we used MPI in a hybrid physiologically-based pharmacokinetic (PBPK) model to account for regional changes in drug distribution that were observed. Subsequently, the PBPK models were linked to a pharmacodynamic model that could account for the variability observed in pERK levels. Together, our tumor sectioning protocol enabled integration of the intratumoral pharmacokinetic/pharmacodynamic variability of gefitinib and immunohistochemical indices followed by the construction of a predictive PBPK/pharmacodynamic model. These types of models offer a mechanistic basis to understand tumor heterogeneity as it impacts the activity of anticancer drugs.

Cytotoxic effect of menadione and sodium orthovanadate in combination on human glioma cells

Gliomas are the most common primary brain tumor, and their treatment is still a challenge. Here, we evaluated the antiproliferative effect of a novel combination of two potent oxidative stress enhancers: menadione (M) and sodium orthovanadate (SO). We observed both short-term and prolonged growth inhibitory effects of M or SO alone as well as in combination (M:SO) on DBTRG.05MG human glioma cells. A stronger antiproliferative effect was observed in the short-term proliferation assay with the M:SO combination compared to either investigated agent alone. In the long-term proliferation assay, a 10-day exposure to M:SO at concentrations of 10 μM:17.5 μM or 17.5 μM:10 μM was enough to kill 100% of the cells; no cell regrowth was observed after re-incubation in drug-free media. When used in combination, the single concentration of M and SO could be decreased by 2.5- to 5-fold of those used for each experimental drug alone and still obtain a similar antiproliferative effect. The underlying molecular mechanism was investigated by co-incubating M:SO with dithiothreitol (DTT) and genistein. Both substances partially neutralized the effects of the M:SO combination, showing additive effects. This observation suggests a role of oxidative stress and tyrosine kinase stimulation in the M:SO cytotoxic effect. Our results indicate that M:SO combination is an attractive alternative for glioma treatment that encourages further study. The neutralizing effects of genistein and DTT reveal a possibility for their use in the minimization of potential M:SO systemic toxicity.

The use of convection-enhanced delivery with liposomal toxins in neurooncology

Liposomes have long been effective delivery vehicles for transport of toxins to peripheral cancers. The combination of convection-enhanced delivery (CED) with liposomal toxins was originally proposed to circumvent the limited delivery of intravascular liposomes to the central nervous system (CNS) due to the blood-brain-barrier (BBB). CED offers markedly improved distribution of infused therapeutics within the CNS compared to direct injection or via drug eluting polymers, both of which depend on diffusion for parenchymal distribution. This review examines the basis for improved delivery of liposomal toxins via CED within the CNS, and discusses preclinical and clinical experience with these therapeutic techniques. How CED and liposomal technologies may influence future neurooncologic treatments are also considered.

Heterogeneous blood-tumor barrier permeability determines drug efficacy in experimental brain metastases of breast cancer

PURPOSE

Brain metastases of breast cancer appear to be increasing in incidence, confer significant morbidity, and threaten to compromise gains made in systemic chemotherapy. The blood-tumor barrier (BTB) is compromised in many brain metastases; however, the extent to which this influences chemotherapeutic delivery and efficacy is unknown. Herein, we answer this question by measuring BTB passive integrity, chemotherapeutic drug uptake, and anticancer efficacy in vivo in two breast cancer models that metastasize preferentially to brain.

EXPERIMENTAL DESIGN

Experimental brain metastasis drug uptake and BTB permeability were simultaneously measured using novel fluorescent and phosphorescent imaging techniques in immune-compromised mice. Drug-induced apoptosis and vascular characteristics were assessed using immunofluorescent microscopy.

RESULTS

Analysis of over 2,000 brain metastases from two models (human 231-BR-Her2 and murine 4T1-BR5) showed partial BTB permeability compromise in greater than 89% of lesions, varying in magnitude within and between metastases. Brain metastasis uptake of ¹⁴C-paclitaxel and ¹⁴C-doxorubicin was generally greater than normal brain but less than 15% of that of other tissues or peripheral metastases, and only reached cytotoxic concentrations in a small subset (∼10%) of the most permeable metastases. Neither drug significantly decreased the experimental brain metastatic ability of 231-BR-Her2 tumor cells. BTB permeability was associated with vascular remodeling and correlated with overexpression of the pericyte protein desmin.

CONCLUSIONS

This work shows that the BTB remains a significant impediment to standard chemotherapeutic delivery and efficacy in experimental brain metastases of breast cancer. New brain permeable drugs will be needed. Evidence is presented for vascular remodeling in BTB permeability alterations.

Modern multidisciplinary management of brain metastases

Ideal management of brain metastases (BMs) requires simultaneous control of the existing brain metastasis (local brain control), prevention of future BMs (distant brain control), and control of the systemic cancer (systemic control). Available tools include whole brain radiation therapy (WBRT), surgery, stereotactic radiosurgery (SRS), and systemic therapies, such as chemotherapies, biologic agents, and radiosensitizing agents. Selecting the combination of these tools is highly individualized and is impacted by numerous factors involving the tumor, patient, provider, and evolving evidence. Historically, patients received WBRT, either alone or with local treatments (surgery or SRS). However, concern about the effects of WBRT, coupled with improvements in local control and survival in select patients, with the combination treatment, has led to a reconsideration of the role of WBRT. Additionally, there have been advancements in the efficacy and tolerance of systemic therapies and clarification regarding the relative risks and symptoms of tumor recurrence versus treatment complications. Thankfully, individualizing modern multidisciplinary management for patients with BMs is being aided by numerous recently completed, ongoing, and planned prospective series.

Plasma and cerebrospinal fluid pharmacokinetics of topotecan in a phase I trial of topotecan, tamoxifen, and carboplatin, in the treatment of recurrent or refractory brain or spinal cord tumors

PURPOSE

This study was designed to ascertain the dose-limiting toxicities (DLT) and maximally tolerated doses of the combination of fixed-dose tamoxifen and carboplatin, with escalating doses of topotecan, and to determine the pharmacokinetics of topotecan in the plasma and cerebrospinal fluid.

METHODS

Tamoxifen 100 mg po bid, topotecan 0.25, 0.5, 0.75, or 1.0 mg/m(2)/d IV, administered as a 72 h continuous infusion on days 1-3, followed by carboplatin AUC = 3, IV on day 3. Cycles were repeated every 4 weeks.

RESULTS

Seventeen patients received 39 cycles of treatment: median 2, (range 1-5). The tumors included glioblastoma (6), anaplastic astrocytoma (2), metastatic non-small cell (3), small cell lung (2), and one each with medulloblastoma, ependymoma, and metastatic breast or colon carcinoma. The median Karnofsky performance status was 70% (range 60-90%) and age: 52 (range 24-75). Eleven patients were female and six male. Toxicities included thrombocytopenia (2), neutropenia without fever lasting 6 days (1), DVT (2), and emesis (1). Topotecan levels, total and lactone, were measured prior to the end of infusion in plasma and cerebrospinal fluid (CSF). At 1.0 mg/m(2)/d, the median CSF/plasma ratio was 19.4% (range 15.1-59.1%). The total plasma topotecan in two pts with DLTs was 4.63 and 5.87 ng/ml, in three without DLTs at the same dose level the mean total plasma topotecan was 3.4 ng/ml (range 3.02-3.83). Plasma lactone levels were 33% of the total; CSF penetration was 20% of the total plasma levels. 4/8 pts with high-grade gliomas had stable disease (median: 3 cycles (range 2-5)). Two had minor responses. One patient with metastatic non-small cell and one with small cell lung cancer had objective PRs.

CONCLUSIONS

The recommended phase II doses are: tamoxifen 100 mg po bid, topotecan 0.75 mg/m(2)/d IV continuous infusion for 72 h, followed by carboplatin AUC = 3 IV on day 3. Measurable topotecan levels, both total and lactone, are observed in the CSF.

Recent progress towards development of effective systemic chemotherapy for the treatment of malignant brain tumors

Systemic chemotherapy has been relatively ineffective in the treatment of malignant brain tumors even though systemic chemotherapy drugs are small molecules that can readily extravasate across the porous blood-brain tumor barrier of malignant brain tumor microvasculature. Small molecule systemic chemotherapy drugs maintain peak blood concentrations for only minutes, and therefore, do not accumulate to therapeutic concentrations within individual brain tumor cells. The physiologic upper limit of pore size in the blood-brain tumor barrier of malignant brain tumor microvasculature is approximately 12 nanometers. Spherical nanoparticles ranging between 7 nm and 10 nm in diameter maintain peak blood concentrations for several hours and are sufficiently smaller than the 12 nm physiologic upper limit of pore size in the blood-brain tumor barrier to accumulate to therapeutic concentrations within individual brain tumor cells. Therefore, nanoparticles bearing chemotherapy that are within the 7 to 10 nm size range can be used to deliver therapeutic concentrations of small molecule chemotherapy drugs across the blood-brain tumor barrier into individual brain tumor cells. The initial therapeutic efficacy of the Gd-G5-doxorubicin dendrimer, an imageable nanoparticle bearing chemotherapy within the 7 to 10 nm size range, has been demonstrated in the orthotopic RG-2 rodent malignant glioma model. Herein I discuss this novel strategy to improve the effectiveness of systemic chemotherapy for the treatment of malignant brain tumors and the therapeutic implications thereof.

Pleomorphic malignant fibrous histiocytoma: response of bone, lung, and brain metastases to chemotherapy

We present a case of pleomorphic malignant fibrous histiocytoma arising from the left forearm in a 45-year-old man who had undergone resection and radiotherapy for a tumor 3 years previously. At his first visit, he had multiple lung and bone metastases. Although these metastases responded well to systemic chemotherapy, brain metastases newly appeared and caused the death of the patient. These findings demonstrate that individual sarcomatous metastatic organs exhibit different sensitivities to chemotherapy. The mechanism of this phenomenon is discussed with a review of previous reports. It is suggested that the blood-brain barrier may play an important role in sensitivity to chemotherapy.

Effect of blood brain barrier permeability in recurrent high grade gliomas on the intratumoral pharmacokinetics of methotrexate: a microdialysis study

PURPOSE

Determining whether potentially therapeutic drug exposure is achieved within brain tumors in an exploratory clinical investigation would provide a rational basis for selecting agents for evaluation in phase II trials. This study investigated the use of microdialysis to assess intratumoral drug distribution in patients with recurrent high grade gliomas (HGG).

PATIENTS AND METHODS

Microdialysis catheters were placed during surgery for residual HGG 1-day before giving methotrexate (MTX) 12-g/m(2) by 4-h i.v. infusion. MTX was measured by Liquid Chromatography/Mass Spectrometry (LC/MS) in plasma and microdialysate during the infusion and for 24-h thereafter. Blood brain barrier (BBB) permeability of tissue in which the microdialysis probe was located was determined by digitally fusing brain CT and contrast enhanced MRI images.

RESULTS

The microdialysis probe was located in contrast enhancing tumor in two patients and nonenhancing tissue in two others. Cerebral drug penetration, as indicated by the ratio of the area under the MTX concentration-time curves in brain extracellular fluid and plasma, was considerably greater in contrast enhancing tumor (0.28-0.31) than nonenhancing tissue (0.032-0.094). Nevertheless, MTX concentrations in ECF exceeded 2-microM, the average concentration for 50% cell kill against glioma cell lines in vitro, for 20-26 h in both regions of the tumor.

CONCLUSIONS

Microdialysis is a very informative technique for characterizing the intratumoral pharmacokinetics of drugs, such as MTX, that do not freely penetrate the BBB. Establishing the catheter probe location relative to areas of BBB disruption is required to properly assess the significance of microdialysis data in this context.

Intracarotid delivery of drugs: the potential and the pitfalls

The major efforts to selectively deliver drugs to the brain in the past decade have relied on smart molecular techniques to penetrate the blood-brain barrier, whereas intraarterial drug delivery has drawn relatively little attention. Meanwhile, rapid progress has been made in the field of endovascular surgery. Modern endovascular procedures can permit highly targeted drug delivery by the intracarotid route. Intracarotid drug delivery can be the primary route of drug delivery or it could be used to facilitate the delivery of smart neuropharmaceuticals. There have been few attempts to systematically understand the kinetics of intracarotid drugs. Anecdotal data suggest that intracarotid drug delivery is effective in the treatment of cerebral vasospasm, thromboembolic strokes, and neoplasms. Neuroanesthesiologists are frequently involved in the care of such high-risk patients. Therefore, it is necessary to understand the applications of intracarotid drug delivery and the unusual kinetics of intracarotid drugs.

Targeting the brain: rationalizing the novel methods of drug delivery to the central nervous system

Drug delivery to the brain has remained one of the most vexing problems in translational neuroscience research. This review rationalizes the strategies to target drugs to the brain. Factors such as the speed of intervention, the scale of intervention, the state of BBB, and the permissible risks, will all be critical in deciding how best to deliver drugs to a target site in the brain for a specific clinical situation.

Targeted delivery of doxorubicin to the rat brain at therapeutic levels using MRI-guided focused ultrasound

The clinical application of chemotherapy to brain tumors has been severely limited because antitumor agents are typically unable to penetrate an intact blood-brain barrier (BBB). Although doxorubicin (DOX) has been named as a strong candidate for chemotherapy of the central nervous system (CNS), the BBB often prevents cytotoxic levels from being achieved. In this study, we demonstrate a noninvasive method for the targeted delivery of DOX through the BBB, such that drug levels shown to be therapeutic in human tumors are achieved in the normal rat brain. Using MRI-guided focused ultrasound with preformed microbubbles (Optison) to locally disrupt the BBB and systemic administration of DOX, we achieved DOX concentrations of 886 +/- 327 ng/g tissue in the brain with minimal tissue effects. Tissue DOX concentrations of up to 5,366 +/- 659 ng/g tissue were achieved with higher Optison doses, but with more significant tissue damage. In contrast, DOX accumulation in nontargeted contralateral brain tissue remained significantly lower for all paired samples (p < 0.001). These results suggest that targeted delivery by focused ultrasound may render DOX chemotherapy a viable treatment option against CNS tumors, despite previous accessibility limitations. In addition, MRI signal enhancement in the sonicated region correlated strongly with tissue DOX concentration (r = 0.87), suggesting that contrast-enhanced MRI could perhaps indicate drug penetration during image-guided interventions. Our technique using MRI-guided focused ultrasound to achieve therapeutic levels of DOX in the brain offers a large step forward in the use of chemotherapy to treat patients with CNS malignancies.

Increased permeability of the blood-brain barrier to chemotherapy in metastatic brain tumors: establishing a treatment paradigm

There is no accepted standard of care for the chemotherapy treatment of metastatic brain tumors, which has been generally limited to lipophilic alkylators, which may not have efficacy against the tumor that metastasized to the brain. More than 50% of chemotherapy agents are natural product drugs, which are rarely used in the treatment of metastatic brain tumors because they are thought to not cross the blood-brain barrier (BBB). A major protein constituent in the BBB is P-glycoprotein (P-gp), which pumps natural product chemotherapy drugs and toxins out of the CNS. However, P-gp expression in the neovasculature of metastatic brain tumors is similar to the P-gp expression in the neovasculature of the primary, extracranial tumor. In contrast, gliomas have higher P-gp expression in their neovasculature, similar to the greater intrinsic expression of P-gp in normal brain vasculature. This decreased immunohistochemical expression of P-gp in the neovasculature of metastatic tumors, as well as our recent pharmacologic demonstration of increased tissue concentrations of paclitaxel in metastatic brain tumors compared with gliomas, support the idea that the choice of chemotherapy agents should be based on the histologic origin of the metastatic brain tumor and not on the lipophilicity of the drug. Our hypothesis is that metastatic brain tumors from tumors with intrinsically low P-gp expression (eg, lung, melanoma, and untreated breast) may be more permeable to natural product chemotherapy drugs than gliomas. This information could lead to a paradigm shift in the use of natural product drugs for metastatic brain tumors.

Chemotherapy and targeted molecular therapies for brain metastases

As therapy for systemic cancers improves, an increasing number of patients are developing brain metastases. Although conventional therapy with surgery, radiation therapy and radiosurgery has improved the outcome of a significant number of patients, many develop multiple lesions that are not amenable to standard treatments. In this review, the current role of chemotherapy and targeted molecular agents for brain metastases is summarized and future directions are discussed.